I recently started my preparations and seem to be struggling a lot in Data sufficiency questions, particularly those on numbers. It would be great if any one could recommend a good source for working on problems such as this one.

Question:

p and q are positive integers. What is the highest prime factor of the product of p and q?

(1) The GCD of p and q is 5

(2) The LCM of p and q is 105

Can some one provide a detailed explanation. I have obtained an answer but its incorrect.